The present invention relates to a protective relay system of a distance measurement type.
Protective relay systems of a distance measurement type are classified into those determining the distance in accordance with an impedance in a steady state and those utilizing a differential equation which holds even in a transient state. The protective relay systems of the former class have been widely adopted for a long time because stable characteristics are obtained. But they have encountered a problem in recent years when distortions in the voltage and current are greater, and filters have to be inserted to remove the distortions and this in turn causes a delay in the response due to delay in the output of the filter. For this reason, the protective relay systems of the latter class are now drawing attentions. Examples of the protective relay system belonging to this class are shown in the following publications:
(i) Japanese Patent Application Publication No. 31747/1978, "A system for measuring an impedance component in an electric power system".
(ii) IEEE Paper F77 052-4, W. D. Breingan et al, "The Laboratory Investigation of a Digital System for the Protection of Transmission Lines".
These systems both determine the inductance L of an electric power system, e.g., a transmission line, by solving a differential equation EQU v=L(di/dt)+Ri (1)
where v, i and R respectively represent the voltage, the current and the resistance of the transmission line, by utilizing an approximate expression: EQU (di/dt).apprxeq.(i.sub.1 -i.sub.0)/(t.sub.1 -t.sub.0) (2)
where i.sub.0 and i.sub.1 represent the values of the current i at different time points t.sub.0 and t.sub.1 (having one sampling interval between them).
The approximation by the equation (2) has a sufficiently high accuracy as far as the variation in the current i is slow compared with the difference t.sub.1 -t.sub.0, i.e., one sampling interval. In other words, a sufficient accuracy is ensured for the fundamental frequency component but the error for the second harmonic or the like is considerable. It is necessary to improve the frequency characteristics in order to attain a sufficiently high accuracy over a wide frequency range without removing the frequency components of these regions by the use of a filter.